Milk of livestock animals, of which typical examples are cow, sheep, and goat, may not be sterile, and may be contaminated with certain microorganisms due to diseases or environment. In particular, it is known that animals with a disease caused by infection of a microorganism in the udder often discharge a lot of the microorganisms into milk. Typical diseases of livestock animals caused by infection of a microorganism include mastitis.
Mastitis is inflammation of the laticifer system or milk gland tissue, and it is caused largely by invasion, colonization, and proliferation of a microorganism in the udder. Although many kinds of animals contract mastitis, it is said that, especially concerning cow's mastitis in dairy cows, 15 to 40% of the whole dairy cows contract mastitis, and thus it is one of the extremely important diseases for dairy farmers. If a dairy cow contracts mastitis, not only the milk synthesis function is inhibited to result in reduction of lactation amount, or even stop of lactation as the case may be, but also enormous economical losses are imposed on dairy farmers, such as cost of medical treatment and penalty concerning milk price due to degradation of milk quality. Furthermore, it also increases the labor of dairy farmers, since, for example, milking of teats suffering from mastitis must be separately performed for preventing infection.
Mastitis is caused by infection of various microorganisms. Among the causative bacteria, staphylococci, especially Staphylococcus aureus, are known as causative bacteria of intractable mastitis, and it is also known that they are transmitted to other dairy cows via milking machines, and so forth.
As the method for detecting staphylococci in milk, cultivation-based methods are widely used. Since the cultivation-based methods require several days for obtaining a result, they are not suitable for quick identification of causative bacteria. In contrast, identification methods based on an antigen-antibody reaction using an antibody directed to an ingredient specific to a causative bacterium, especially the immunochromatographic method, can provide the result in several tens of minutes, and therefore they are widely used as quick and convenient inspection methods (for example, Patent document 1). The inventors of the present invention have examined use of an immunochromatographic method also as a method for detecting a substance contained in milk of livestock animals (Patent document 2).
In immunological measurement methods such as the immunochromatographic method, it is necessary to lyse bacterial cells to release antigens from the inside of the cells to the outside of the cells. As the method for lysing Staphylococcus aureus cells, there are known a method of using a lytic enzyme, lysostaphin (Patent documents 1 and 3), a method of using a treatment with a lytic enzyme and/or a bacteriocin having a bacteriolytic activity, a surfactant, and a protein denaturant (Patent document 4), and a method of using a treatment with a lysis reagent containing a lytic enzyme, a nonionic surfactant, and a chelating agent (Patent document 5). However, the aforementioned patent documents describe use of cultured cell suspension, and do not describe lysis effects in a high protein and high fat content solution such as milk.